What is sleep debt?
Sleep debt is the gap between the sleep your body needs and the sleep it actually gets, accumulated over time. Sleep five nights at 6.5 hours when you need 8, and you're carrying 7.5 hours of debt into the weekend, roughly a full night.
Debt builds silently: after a few short nights you stop noticing how impaired you are, even though reaction time, mood, and focus keep degrading. The good news is that debt can be repaid gradually. Going to bed 30–60 minutes earlier for a week beats one heroic 12-hour weekend sleep, which mostly disrupts your rhythm.
Not sure what your nightly need is? Check the recommended hours for your age, then use the sleep calculator to pick a bedtime that gets you there in full cycles.